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Evaluation of power output for ﬁxed and two-axis tracking PVarrays
Ahmet S

enpinar
a,
⇑
, Mehmet Cebeci
b
a
College of Technical Sciences, Department of Electronics Technology, Firat University, Elazig, Turkey
b
Department of Electrical-Electronics Engineering, Firat University, Elazig, Turkey
a r t i c l e i n f o
Article history:
Received 21 January 2011Received in revised form 7 July 2011Accepted 24 July 2011Available online xxxx
Keywords:
Two-axis PVsystemSun-tracking systemComputer
a b s t r a c t
This study compares the performance of two PVmodules, one ﬁxed and the other ﬁtted with a two-axistracking systemwhich enables the PVcollector to move and be controlled to followthe Sun’s radiation. Acomputer unit was employed to monitor the solar radiation exposure of both systems and to control themovement of the solar tracking PVmodule. Throughout the day, the results obtained from the ﬁxed andtracked systems were collected on a DAQ card installed on an online computer. The two motors of thetrackingsystemwereconnectedtooutputfromthecomputerthatwasusedtofollowthesun.Themove-ment of the two-axis tracking PVmodule was compared to mathematical calculations of the sun’s posi-tion from sunrise to sunset. The amount of solar radiation received on the surface of the module andhence its output power varied according to the time and the place where the experimental systemwas mounted. Overall, it was found that the daily output power of the tracking module was 13–15%higher than the ﬁxed module.

2011 Elsevier Ltd. All rights reserved.
1. Introduction
Recent technological developments offer us the opportunity togenerate clean, continuous and inexpensive electricity from arenewablesource,solarenergy.Theoutputenergyofasolarenergysystem depends on various factors including the amount of thesun’s radiation entering the system. Tracking systems can help toorient the solar collector to optimize its alignment with the maxi-mum incident beam of the sun’s radiation. Different sun-trackingsystems have been developed to track the sun’s movement acrossthe sky [1–13]. Chang [14] used the Julian dating system to calcu-
late the Sun’s apparent position. This method calculates both theduration of sunshine on any given day and the optimal installationangle of a ﬁxed solar collector for different time periods and lati-tudes in the northern hemisphere.In a previous study two identical PVarrays were compared, oneﬁtted with a microprocessor-controlled, two-axis solar trackingsystem, while the other was ﬁxed [15]. Solar radiation inputs andelectricalpoweroutputsweremeasuredforeachunitovera1-yearperiod. The array ﬁtted with the solar tracking system performedsigniﬁcantly better. It was demonstrated that the performance of a solar radiation collector is affected by its orientation and tilt an-gle(
b
) withthe horizontal plane [16]. Theseparameters determinethe amount of solar energy received at the surface of the collector.It should be noted that solar cells have non-linear current–voltagecharacteristics that require the maximum power point tracking(MPPT) of the systemto be calculated both in terms of the amountof insolation at the surface of the array and the optimal voltageloadingofit’ssolarcells.Aconverterisusedtoregulatethevoltageof the solar array in accordance with a reference voltage that cor-responds to the maximum point and array voltage. A neural net-work with a training algorithm has been utilized to keep thereference voltage under the control of the maximum power point[17,18]. Various other studies have also looked at the dynamicsof maximum power point tracking systems [19–25].In this study, electricity generation efﬁciencies were comparedin a ﬁxed and a two-axis solar tracking array. The ﬁxed array waspositionedwithaspeciﬁcﬁxedangleoftiltthatwouldutilizemostof the available sunlight at the geographical location for the study.The second array would track the sun’s movement by day. Twocomputer-controlled motors were used to enable the array to bemoved in east–west and north–south planes so that available sun-lightwouldbeutilizedtothemaximumeachday.ThetwoPVarraysystems were simultaneously operated and their current and volt-agevalueswererecordedonanonlinecomputer.Thepoweroutputof both systems was calculated according to the data obtained,graphicsdrawnandtheresultswerethencompared[26]. Thesolartracking array output power gain showed a considerable increaseduring the early hours of the morning and the late hours of theevening. The amount of increase was about 1–2% during mid-dayhours. The most advantage of the two-axis solar tracking arraywasitsabilitytomaintainitspositionvalueswithoutbeingaffectedbycloudsorandotherenvironmentalconditions(wind,storm,rain,dust, pollution, etc.). While the solar tracking system is more
0306-2619/$ - see front matter

2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.apenergy.2011.07.043
⇑
Corresponding author. Tel.: +90 424 2370000/4384; fax: +90 424 2188947.
E-mail addresses:
asenpinar@gmail.com (A. S

enpinar), mcebeci@ﬁrat.edu.tr(M. Cebeci).Applied Energy xxx (2011) xxx–xxx
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Please cite this article in press as: S

enpinar A, Cebeci M. Evaluation of power output for ﬁxed and two-axis tracking PVarrays. Appl Energy (2011),doi:10.1016/j.apenergy.2011.07.043
expensive and complex than a ﬁxed mounted one, it can be mademore economical if additional PVmodules are connected in seriesor parallel.
2. Mathematical model of the system
2.1. PV Cells
A PV cell is a specialized semiconductor material with a
p
–
n
junction. It converts sunlight into electricity through a basic pro-cess called photovoltaic effect. The energy generated by the cellis in direct proportion with the visible light it has been exposedto. Additionally, conversion efﬁciency also depends on extendingthe plane. Fig. 1 shows the
I
–
V
characteristics of a typical cell[27].Theamountofthecurrentandthevoltagechangesdependingon the amount of sunlight shining on the cell. Then, the
I
–
V
equa-tion is:
I
¼
I
l

I
o
e
ð
qV
Þ
=
ð
kT
Þ

1
 
ð
1
Þ
where
I
l
is the component of the PV cell current due to photons,electrical load (
q
=1,6

10

19
C),
k
=1.38

10

23
j/K (Boltzmanconstant) and
T
is the cell temperature in Kelvin.Fig. 1 shows that a PV cell has a voltage and current limitation.This indicates that the cell will not be damaged by open or shortcircuits.ToidentifyaPVcell’sshortcircuitcurrent,
V
=0iswritten.Then,
I
sc
=
I
l
. Thus, if the cell current is known under Standard TestConditions (STC); that is for
G
o
=1kw/m
2
and AM1.5, then the cellcurrent in any other
G
radiation is calculated by:
I
l
ð
G
Þ
¼
G
=
G
o
ð Þ
I
l
ð
Go
Þ
ð
2
Þ
In order to identify the cell’s open circuit voltage, the cell cur-rent is set to zero and the equation in 1 is solved for
V
oc
and theresult is:
V
oc
¼
kT q
 

ln
I
l
þ
I
o
I
o
 

kT q
 

ln
I
l
I
o
 
because
I
l

I
o
ð Þ ð
3
Þ
Whiletheshortcircuitcurrentof thecell variesindirectproportionwith the amount of illumination, open circuit voltage is only loga-rithmically dependent on illumination.
2.2. Fixed system
Thesearesystemswherethearrayof solar cellsis placedwithaspeciﬁcﬁxedtilt.Tiltanglechangesaccordingtotheseasonandre-gion. Generally, PVsystems in the northern hemisphere aremounted facing due south with a certain angle. A variety of differ-entvaluesforthetiltanglehavebeensuggested.Somestudieshaveused angles such as, Ø+20

[28], Ø+(10
?
30)

[29], Ø+10

[30],Ø

10

[31], whereas other researchers suggest two values for thetilt angle, such as Ø±20

[32], Ø±8

[33], Ø±5

[34], where Ø isthelatitudeangleoftheregion,‘‘+’’forwinter,and‘‘

’’forsummer.For optimal performance on any given day, a ﬁxed array should bemountedonthegroundtohaveahorizontalangleof(Ø

d
)

.Here,
d
isthedeclinationangleknownastheanglebetweenthedirectionofthesunandequatorplane.Here,thebestaveragetiltforsummer,winter and optimum yearly performance should be mounted at(Ø

15)

, (Ø+15)

and with a (0.9.Ø)

tilt angle of the array,respectively [27].The declination angle can be calculated from the equation usedby Copper [35,36];
d
¼
23
;
45

sin
ð
360
ð
284
þ
n
ÞÞ
365
 

ð
4
Þ
where
n
is the day of the year. For horizontal surfaces, the angle (
h
)of incidence is the zenith angle of the sun,
h
z.
For this situation,
cos
h
z
¼
cos
d

cosØ

cos
x
þ
sin
d

sinØ
ð
5
Þ
where
x
is the solar hour angle. For 24h time, the solar hour isdetermined by (Senpinar) formula as follows:
x
¼
hour

60
þ
minute
ð Þ
720
ð Þ
=
4

ð
6
Þ
Thesolarnooniscalculated, forasouthfacingslopeinthenorthernhemisphere [35],
h
noon
¼j
Ø

d

b
j ð
7
Þ
where
b
=0, theangleofincidenceisthezenithangle, whichforthenorthern hemisphere
h
noon
¼j
Ø

d
j ð
8
Þ
Solar altitudeangle(
a
s
) is theanglebetweenthehorizontal andtheline to sunandit is the complement of the zenithangleto 90

. Thatis
h
z
þ
a
s
¼
90

ð
9
Þ
and due to trigonometric function,
cos
h
z
¼
sin
a
s
and sin
h
z
¼
cos
a
s
ð
10
Þ
The solar azimuth angle
c
s
changes values in the range of 180

to

180

. It is knowas the angular displacement fromsouth of theprojection of beam radiation on the horizontal plane. For north orsouth latitudes between 23.45

and 66.45

,
c
s
will be between 90

and

90

.Tocalculate
c
s
,weneedknowthesunposition.Ageneralformula for
c
s
, from Braun and Mitchell [37], is conveniently writ-ten in terms of
c
s
, a pseudo surface azimuth angle in the ﬁrst orfourth quadrant:
c
s
¼
a
1

a
2

c
0
s
 
þ
a
3
 ð
1
ð
a
1

a
2
ÞÞ
=
2
ð Þ
180
ð
11
Þ
where
sin
c
0
s
¼ ð
sin
x

cos
d
Þ
=
sin
h
z
ð Þ ð
12
Þ
a
1
¼
1if
j
x
j
x
ew
else
a
1
¼
1if
j
x
j
>
x
ew
;
ð
13
Þ
a
2
¼
1if Ø

d
ð Þ
0else
a
2
¼
1if Ø

d
ð Þ
<
0
;
ð
14
Þ
a
3
¼
1if
x

0else
a
3
¼
1if
<
0
:
ð
15
Þ
cos
x
ew
¼
tan
d
tan
/
ð
16
Þ
and cos
x
s
¼ 
sin
/

sin
d
cos
/

cos
d
 
¼ 
tanØ

tan
d
ð Þ ð
17
Þ
where
x
ew
is the sunrise hour angle and
x
s
is the sunset hour an-gle.Theﬁxedsystemusedinthe experimental studywas illustratedinFig.2.Itcomprisedasolararray,measurementgroup,chargereg-
0.700.10.20.30.40.50.60123
Cell voltage (V)
C e l l c u r r e n t ( A )
R e a l c e l l
1 kw/m
2
750 w/m
2
500 w/m
2
250 w/m
2
Fig. 1.
I
–
V
characteristics of the real and ideal PV cells under different illuminationlevels.2
A. S

enpinar, M. Cebeci/Applied Energy xxx (2011) xxx–xxx
Please cite this article in press as: S

enpinar A, Cebeci M. Evaluation of power output for ﬁxed and two-axis tracking PVarrays. Appl Energy (2011),doi:10.1016/j.apenergy.2011.07.043
ulator, battery, inverter and load converter. The numerical resultsobtained from the system were stored online on a computer. Somecomponents in the systems (42W array, battery, charge regulatorand inverter) were identical. The systems were designed to enableenergy received from the sun to be stored in batteries so that itcould be retained for use during cloudy weather or at night. DCloads could be directly supplied from the batteries. If the load wasAC, an inverter enabled the conversion to DC voltage of appropriateamplitudeandfrequencyforbatterystorage.Inexperimentalstudy,500W inverters were used.
2.3. Tracking system
Most tracking arrays follow the sun in prescribed ways to min-imizetheangleofincidenceoutofbeamradiationontheirsurfacesandsomaximizetheincidentbeamradiation.Trackingsystemsareclassiﬁedbytheir motions. Rotationcanbe about a single axis or itcan be about two axis [35]. The system used in this study consistsof solar array which moves in two-axis (Fig. 3a). It is controlled bytwo different motors, one tracking in a north–south axis and theother in an east–west axis. If the array is moved to track the sunaccurately, the angle of the light incident to the array will be nor-mal. Thus, voltage and current obtained from the array will bemaximized and solar energy will be utilized more efﬁciently. Thecontrol of themotorsinthesystemwasprovidedbythecomputer.Latitude that determines the region of the PVsystem, date, timevalues were loaded onto the computer as data. Mathematical cal-culations of solar angles related to the region were automaticallygenerated by a program on the computer. The computer also con-trolled the motors so that the array would be slaved to replicatethe calculated values, allowing for it to track the movement of the sun throughout the day.Foraplanerotatedaboutahorizontaleast–westaxiswithasin-gle daily adjustment so that the beam radiation is normal to sur-face at noon each day;
cos
h
¼
sin
2
d
þ
cos
2
d

cos
x
ð
18
Þ
The tilt of this surface will be ﬁxed for each day;
b
¼
Ø

d
j j ð
19
Þ
The surface azimuth angle (
c
) for a day will be 0

or 180

depending on the latitude and declination;
If Ø

d
ð Þ
>
0
;
c
¼
0

;
ð
20
Þ
If Ø

d
ð Þ
<
0
;
c
¼
180

ð
21
Þ
where
c
is the deviation of the projection on a horizontal plane of the normal to the surface from local meridian. For a plane rotatedabout horizontal east–west axis with continuous adjustment tominimize the angle of incidence [35];
cos
h
¼
1

cos
2
d

sin
2
x
 
1
=
2
ð
22
Þ
The tilt of this surface is given by
ChargeRegulatorBatteryMeasurement/ AmplifierComputerLoadArrayInverter
Fig. 2.
The block scheme of ﬁxed array.
ChargeRegulatorBatteryMeasurement/ AmplifierComputerMotorsLoadMechanicalSystemEncoderInverterArray
Fig. 3a.
The block scheme of tracking array.
A. S

enpinar, M. Cebeci/Applied Energy xxx (2011) xxx–xxx
3
Please cite this article in press as: S

enpinar A, Cebeci M. Evaluation of power output for ﬁxed and two-axis tracking PVarrays. Appl Energy (2011),doi:10.1016/j.apenergy.2011.07.043
tan
b
¼
tan
h
z

cos
c
s
j jð Þ ð
23
Þ
For a plane rotated about a horizontal north–south axis withcontinuous adjustment to minimize the angle of incidence;
cos
h
¼
cos
2
h
z
þ
cos
2
d

sin
2
x
 
1
=
2
ð
24
Þ
The tilt is given by
tan
b
¼
tan
h
z

cos
ð
c

c
s
Þj jð Þ ð
25
Þ
Using a continuous tracking system, sunlight is utilized to themaximum level from sunrise to sunset according to data speciﬁcto that region (Fig. 3b). The system enables the sunlight to runperpendicularly to the array by tracking the array in different an-gles and different directions according to the position of the sunthroughout the day. Accordingly, if this tracking system were tobe located in a different region, appropriate data (latitude, date,etc.) would need to be loaded for that region.The array, measurement group components, charge regulator,batteryandinverterusedinthissystemarethesameasthoseusedin ﬁxed system. Two-axis movement of the system is provided bytwomotors.Athree-phaseACmotorwasusedfortheeast–westaxiswith the movement controlled by output fromthe computer. A DCmotor was used for the north–south axis with the movement con-trolledaccordingtodataprocessedbythecomputersoftware.Thustwo-axis movement of the array was provided by controlling twodifferent motors simultaneously, with computer output. The addi-tional components used to provide the movement and control of thesolartrackingsystemaremotorsenablingthemovement, driv-ers,andanencoderwhichprovidesfeedbacktoanInput/Output(
I
/
O
) data card used to record measurements for storage on thecomputer.
2.3.1. Motors and drivers
The movement of the solar tracking system in the direction of north–south, that is, the tilt angle’s control of the array was pro-vided by the DC motor and its driver. Once the calculations weremade on the computer, the output value was sent via an ampliﬁedelectronic circuit to operate the DC motor. At noon hours, thetracking array’s tilt is nearly equal to the ﬁxed array tilt due tothecontrolofDCmotor.ThereasonforusinganACmotoristopro-vide the required power to accommodate additional arrays. Thecontrolofthemotorwasprovidedbyafrequency-controlleddriverconnected to the computer output. Accordingly, the driver is ableto operate at low frequencies (nearly 1Hz), and the speed of themotor can be decelerated if required. Due to the sensitivity of thecircuit (0.0033

for 1Hz), a gearbox with the ratio of 1/100 is con-nected to the motor output.
Fig. 3b.
The mechanical scheme of tracking array.
Array
Current sensor(LA 25-NP)Input (1-5)Out (6-10)
+15 V-15 V
M
Rm(150 )R(load)
R1 (1.2kΩ/50W)
+HT-HT
+15 V-15 V
M
Rm(220
Ω
)Computerinput 1/ (AI0)Computerinput 2/ (AI1)Voltage sensor(LV 25-P)(+ end)(- end)Fuse(6 A)Charge regulatorBatteryD.C. loads InverterA.C. loads
Fig. 4.
Detailed scheme of experimental connection for ﬁxed system.4
A. S

enpinar, M. Cebeci/Applied Energy xxx (2011) xxx–xxx
Please cite this article in press as: S

enpinar A, Cebeci M. Evaluation of power output for ﬁxed and two-axis tracking PVarrays. Appl Energy (2011),doi:10.1016/j.apenergy.2011.07.043

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